Internal combustion engines combust an air and fuel mixture within cylinders of the engine to produce drive torque. Engines can include a turbocharger that increases torque output by delivering additional air into the cylinders. Some turbochargers are dual stage turbochargers. Dual stage turbochargers have a high pressure stage and a low pressure stage arranged in series. When the engine is operating at low speeds, exhaust flows through the high pressure stage and then through the low pressure stage. By allowing exhaust to flow through the high pressure stage and then the low pressure stage turbo lag may be reduced. As engine speed increases a bypass valve (BPV) may open bypassing the flow of exhaust through the high pressure stage and allowing the exhaust to flow through the low pressure stage.
Some dual stage turbochargers may use a variable geometry turbine (VGT) in the high pressure stage to further reduce turbo lag. The VGT typically has a set of movable vanes to control pressure of the exhaust flowing through the high pressure stage. At low engine speeds when exhaust flow is low, the vanes are partially closed to accelerate the VGT. Accelerating the VGT increases boost pressure delivered to a compressor in the high pressure stage. As engine speed increases, the vanes are opened to slow down the VGT. Slowing down the VGT prevents the boost pressure from reaching excessive levels.
The BPV and the VGT are typically controlled separately. A lookup table indexed by engine speed and torque is typically used to control the position of the BPV. The VGT is typically controlled with a feedback control system. As a result, the BPV may open before the VGT has fully opened, which is undesirable